The deep water oxygen deficit in stratified shallow seas is mediated by diapycnal mixing

Abstract Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer, with predictions that global warming will accelerate this deficiency. Here we integrate novel turbulence timeseries with vertical profiles of water column properties from a seasonal stratified shelf sea to estimate oxygen and biogeochemical fluxes. The profiles reveal a significant subsurface chlorophyll maximum (SCM). We show for the first time that the associated mid-water oxygen maximum supports both upward and downwards O2 fluxes. The upward flux will out-gas to the atmosphere whilst the downward flux will partially off-set the deep water O2 deficit. These new results show that the fluxes are sensitive to both the water column structure and mixing rates indicating that the development of the seasonal O2 deficit is mediated by diapcynal mixing. Analysis of current shear indicate that whilst tidal mixing supports the downward flux, the upwards flux is dominated by wind driven near-inertial shear. Summer storminess therefore plays an important role in determining the development of the deep water O2 deficit.